An important portion of the production of integrated circuits is patterning on the surface of a semiconductor wafer (which is cut into small pieces after processing to make integrated circuit devices). Such patterning defines various regions within an integrated circuit device including ion implantation regions, contact window regions and bonding pad regions, and is generally formed by transferring a geometrical shape pattern of a mask to a thin layer of a photosensitive material (which is called a “resist”) covering a silicon wafer, which forms an integrated circuit device. The pattern of a mask is typically enlarged, and therefore it needs to be reduced for the purpose of projection to a resist.
Currently, the pattern transfer process is generally carried out by photolithography, and radiation energy used for the transfer has optical wavelengths.
If a pattern formed on a resist, i.e., an object having a characteristic is small, the packaging density of circuit elements within an integrated circuit becomes high, and therefore the wavelengths of optical radiation used for transfer need to be short accordingly. It seems that this technology is approaching its limits for usefully using optical radiation necessary for drawing a pattern on a resist in an appropriate manner.
There are recently several other technologies including extreme ultraviolet radiation and electron beams, which it is believed can be used at the time of transferring a geometrical pattern of a mask to a resist layer.
Electron beams (which can be controlled minutely and accurately) recently started being used mainly at the time of producing masks for which optical lithography is now used. Electron beams are used for directly writing a pattern on a resist on a silicon wafer, but such usage is limited to customized circuits for small-quantity production which are sold at high prices.
The difficulty in the use of electron beams used for drawing patterns on a resist at the time of producing an integrated circuit causes a decline in throughputs by such a use, and the difficulty in the use thereof is also caused by the fact that an electron beam exposure system is comparatively expensive. Accordingly, there is generally no likelihood of using an electron beam exposure system, and efforts to develop a commercially available system for such a use have been limited.
In an electron beam lithography system explained in a thesis titled “High Throughput Submicron Lithography with Electron Beam Proximity Printing” (published in September 1984, Solid State Technology, pp. 210-217), electron beams are operated by 10 KV energy (which used to be regarded very low), the thickness of a stencil mask is 2 microns (which is thinner than that of conventional ones), and the interval between a mask and wafer is 5 mm (500 microns) (which is regarded very small). Its electron beams (whose diameter is about 1 mm (1000 microns), which is very small as compared with the entire region of a mask) was raster-scanned with a pair of first deflection coils in such a manner as to traverse the mask. A pair of second deflection coils was used for inclining beams around a pivot point on the mask surface. A silicon wafer containing a membrane having a thickness of 2 microns served as a mask. With such a mask and electrons having 10 KV energy, it was necessary to have a proper metallic absorption layer on the mask in order to block beams that were not directed toward the opening section of the mask. Otherwise, such beams pass through a thin membrane of a thin silicon mask and thereby obfuscates a pattern to be formed on a resist. However, the use of a thicker silicon mask makes it difficult to shorten the (drawing) line width. This is because the aspect ratio of the line width to the mask thickness is too high.
However, it appears that this thesis hardly made any impact on researches in this field such that efforts on such a proximity projection printing system declined after 1984. Research on the electron beam exposure system was rather directed toward a system in which electrons in electron beams have high energy in order to give “rigidity” to electron beams. Beams having rigidity can be controlled well in terms of beam diameter and focused very well, make sharp images and furthermore are not influenced by any leakage electric field. Rigidity is generally related to the energy or speed of electrons within a beam, and as energy increases, the rigidity of the beam goes higher.
For this reason, in commercial use, it is general to use electrons having at least 50 KV energy in order to achieve high resolution. The device using such a beam is generally constituted of a system in which an electron beam from an electron source is formed by focusing and then irradiated to a mask, and a projection system in which a beam passing through a lens and then a mask is reduced and then projected to a resist, wherein the reduction ratio is ⅕ to 1/25.
However, when the density of circuit elements within an integrated circuit increases and thereby the size of an object having a characteristic of a resist pattern is reduced, problems occur for use of high energy beams. Particularly, the proximity effect (which distorts a pattern to be formed as a result of the backscattering of beams emitted from a silicon wafer substrate on the lower side to a resist) increases. This effect becomes more and more problematic as a pattern to be formed on a resist is miniaturized more and more. However, it is known that as the accelerating voltage is increased, forward scattering within a resist is reduced, electrons that are backscattered due to the substrate are scattered over a wide area, and as a result, the amount of dose becomes relatively constant. This does not mean to completely remove the proximity effect though it makes easier to correct the proximity effect. Furthermore, the increase of electron energy means that electrons pass through a resist quickly without releasing much energy, and therefore the resist sensitivity per electron tends to decline. Therefore, as energy is increased, electric current necessary for certain sensitivity becomes higher (i.e., the density of electrons within a beam increases). Moreover, as the density of electrons within a beam increases, the beam is defocused more and more, and the resolution of a pattern is lowered. In addition, as the electric current is increased, a mask, a resist layer and a substrate are heated more, and thereby a projection pattern is distorted accordingly. Hence, in order to maintain necessary accuracy, operating current must be limited. This results in the limitations of throughputs of a device.
In order to address some of those problems, the use of low energy electron beams to draw a pattern on a resist newly attracted some attention for a certain period. Particularly, a thesis titled “Low voltage alternative for electron beam lithography” (J Vac. Cci Tech B 10(6), November/December 1992, pp. 3094-3098) reported that the proximity effect substantially declined by using electrons having relatively low energy within a beam. One of main objectives of the research was to show that even when a very thin resist necessary for using an electron beam having low energy was used, a pattern on a resist could sufficiently be transferred to a substrate. However, such electrons tend to be low in terms of brightness at a low voltage, and therefore it was acknowledged that the application of ultrathin resist layers was difficult.
As a result, it has long been acknowledged that electron beams having low energy are suitable for drawing patterns on a resist and have potential advantages. However, when an electron beam having low energy is raster-scanned in such a manner as to traverse a mask using first deflection coils, the passage of the electron beam is elongated, and thereby the electron beam is influenced by Coulomb interaction (space charge effect). When the current density increases, its influence is further added. As a result, disadvantages excel advantages, and therefore as a method of mass-producing devices, this method has not widely been used yet for commercial use. Nevertheless, substantial development efforts to use low-voltage lithography have recently been made using (1) retarding field electron beam columns, (2) multiple sequence reduced electron beam columns, and (3) the tip ends of multiple sequence scanning tunnel microscopes.
On Oct. 31, 1997, the present inventor filed a patent application in which the title of the invention was “Low Energy Electron Beam Lithography” (which was granted as U.S. Pat. No. 5,831,272 issued on Nov. 3, 1998) (Patent Document 1). Patents were granted for the corresponding applications in Japan and Germany. One embodiment described in U.S. Pat. No. 5,831,272 is a system for patterning a resist on a semiconductor substrate and comprises a 1× stencil mask made of single crystal silicon disposed in the passage of electron beams, and the substrate covered with an electron-sensitive resist within the passage of electron beams and a mask. The resist is thin; the beam accelerating voltage is sufficiently low so that the proximity effect hardly occurs; the beam voltage is sufficiently low so that the mask, the resist and the substrate are hardly heated; and the electron density of a beam is sufficiently low so that the space charge effect hardly occurs. The electron beam accelerating voltage is about 2 KV (in the range of about 1 KV to 5 KV); the resist has a thickness of 100 nm (in the range of 30 nm to 300 nm); the electron beam current is about 3 microamperes (in the range of about 0.3 microamperes to about 20 microamperes); the beam diameter is about 1.0 mm (in the range of about 0.1 mm to about 5 mm); and the mask is a stencil mask having a thickness of about 500 nm (in the range of about 200 nm to 1000 nm). The error in the alignment between a mask and a semiconductor wafer is maintained to be about 15 nm or less. It was known that by 2007, the “LOW ENERGY ELECTRON BEAM LITHOGRAPHY SYSTEM” described in U.S. Pat. No. 5,831,272 would be produced, tested and operated.
FIG. 9 shows a schematic view of an exposure device disclosed in U.S. Pat. No. 5,831,272.
An exposure device 90 in FIG. 9 comprises an electron gun 93 for generating a low-velocity electron beam 92, aperture 94, a condenser lens 95, a pair of main deflectors 96, 97, and a pair of deflectors for fine adjustments 98, 99. The aperture 94 limits the electron beam 91. The condenser lens 95 converges the electron beam 91 to make parallel beams. The main deflectors 96, 97 and the deflectors for fine adjustments 98, 99 are deflection coils, and the main deflectors 96, 97 deflect the electron beams 91 in such a way that the electron beam 91 becomes incident basically perpendicularly on the surface of the stencil mask 90. The electron beam 91 is scanned in such a way as to sweep over the upper surface of the stencil mask. By an electron beam passing through a hole 101 section of the stencil mask 100, a resist 103 on a wafer 102 is exposed.
It was discovered that the use of this system enabled to make the size of an object having a characteristic to less than 1 micron, potentially to about 45 nm. A considerable number of tests were conducted in order to determine the limits of the size of an object having a characteristic while maintaining the level of throughputs high enough to be commercially feasible.